Ion exchange membranes (IEM) are used in electrodialysis (ED) as ion selective membranes and in power sources such as fuel cells as proton conductive separators. Basically, two types of IEMs for use in ED are available: homogeneous, that is composed of a cross-linked polymer, which is chemically treated to bind ion exchange groups onto its skeleton. The other type, heterogeneous IEM, are prepared by random filling of a neutral polymer matrix such as polyethylene by tens of micron sized ion exchange particles, and often they are reinforced with a net made of polymeric material such as polyamide or PES.
Whenever the so-called percolation concentration of the ion exchange powder is surpassed, an ion conductive permselective, heterogeneous membrane results.
The normal ion exchange particle concentration in the matrix required to attain reasonable ion transport through the membrane is 50 to 70% by weight. At these concentrations, the membrane specific electrical conductivity is in the range 4 to 12 mScm−1. In fact, the above concentrations are the maximum possible for the ED application: an attempt to increase ion exchanger concentration beyond this range for obtaining higher conductivity results in loss of membrane strength and reduced shape stability due to increased swelling when exposed to salt solutions. These properties are important when membranes are used in ED stacks since they limit conditions under which these membranes can be operated.
In power sources, such as batteries and fuel cells selective ion conducting membranes are used. For example particularly with fuel cells, the Nafion membrane (homogeneous cation exchanger) is widely used because of its remarkable proton conductance (0.15 mScm−1) and stability in oxidizing conditions. However, the new fuel cell generations require both high proton conductivity and very low permeability to the fuel (for example, hydrogen or methanol fuels). Inorganic or hybrid inorganic-organic composite membranes (both of the heterogeneous type) are today under intensive development for these purposes. One problem encountered in this development is that, as proton conductivity or anionic conductivity in general increase, selectivity to the fuel decreases.
U.S. Pat. No. 3,180,814 discloses a method of increasing conductivity in ion exchange membranes made of synthetic resin through the membrane while the synthetic resin is above its yield point in relation to the magnitude of the electric current. In heat softenable material this may be accomplished by heat development incidental to the passage of the electric current, or by heating of the liquid within which the membrane material is immersed, or by a combination of both. In a material that is not heat-softenable, this may be accomplished by temporary softening, for example by addition of a solvent.
U.S. Pat. No. 5,232,719 describes a process for producing a protein-oriented membrane which is enhanced physically and chemically by orienting protein and cross-linking the oriented protein together.
U.S. Pat. No. 6,287,645 describes a method of forming an oriented film. A target is provided and material from the target is ablated onto a substrate to form a film.
U.S. Pat. No. 5,167,551 describes a process for the production of a heterogeneous ion-exchange membrane, which comprises making a finely powdered ion exchange material with a crystalline polyolefin resin, forming the resultant mixture into a membrane-shaped article and treating this latter with an aqueous solution of an alkali metal or ammonium salt.
U.S. Pat. No. 5,346,924 discloses a method for making a heterogeneous ion exchange membrane, which comprises polyethylene as a binder, incorporates ion exchange resin materials. Said membrane being fabricated using extrusion or other melt processing procedures.
It is a purpose of this invention to produce heterogeneous ion exchange membranes (IEM) for ED or fuel cells wherein the particle or domain threshold concentration needed for conductivity is considerably reduced.
It is another purpose of this invention to provide a method for preparing heterogeneous membranes that are more stable and more conductive than the ones known in the art for the application in electrodialysis (ED) and in power sources such as fuel cells.
It is a further object of this invention to produce such membranes having higher permselectivity, higher conductivity, lower swelling rate and better mechanical stability and processability.
It is a still further purpose of this invention to produce membranes with high proton conductivity and low permeability to fuels such as methanol or hydrogen gas.
It is a still further purpose of this invention to produce such membranes at a reduced production cost.
It is a still further purpose of this invention to produce such membranes having increased ion conductance, in particular proton conductance and permeation rate.
Other purposes and advantages of the invention will appear as the description proceeds.